1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member (photoreceptor) including a charge generation layer and a charge transporting layer on an electroconductive substrate, more particularly, to an electrophotographic photosensitive member suitable for digital photographic methods, and to apparatuses utilizing the said electrophotographic photosensitive member.
2. Description of the Related Art
Over the past few years electrophotographic technology has taken a central role in the world of photocopying, printing, and faxing due to the speed and high quality of the printing obtained.
As is widely known, selenium, selenium tellurium alloys, selenium arsenic alloys and other inorganic photoconductive materials have long been used as electrophotographic photosensitive members in electrophotographic techniques.
However, in terms of cost, productivity, and waste etc., the organic photoconductive materials have excellent advantages over the inorganic based materials. The research in this field has increased to such an extent that now it can be said that the organic photoconductive materials are superior to the inorganic based photosensitive members. In the elementary process of photoconduction, functionally separated layered structures in which the photocharge generation, and charge transport layers are performed in their own separate layers have been developed.
As a result of these structural developments, the breadth of choice of materials has increased and performance has improved remarkably. Organic photosensitive members having functionally separated layered structure organic photosensitive members have become the main photosensitive members of all the electrophotographic photosensitive members.
The charge generation layer for the organic photosensitive members having functionally separated layered structures with pigments capable of generating charge such as quinone pigments, perylene pigments, azo pigments, phthalocyanine pigments, and selenium pigments as well as others have come to be used in practice. The pigments capable of charge generation may be vapor-deposited to form a layer or they may be dispersed in binding resins at high concentration.
At the same time, the charge transport layer is constructed by molecular dispersing low molecular weight compounds such as hydrazone compounds, benzidine compounds, amine compounds, stilbene compounds, and others having charge transporting ability in an insulating resin.
The photoreceptors used by the electrophotographic copying machines in the analogue system, in which an image of an original is focused on the photoreceptor to effect an exposure, need to have good half-tone reproducibility through optical density gradation. In order to achieve satisfactory half-tone reproducibility, a photosensitive member with photo-induced potential decaying properties is used (FIG. 1). In other words, a photosensitive member causing potential decaying characteristics proportional to the amount of exposure is demanded. From now on, this-type of photosensitive member is referred to as a J-type photosensitive member.
The above mentioned inorganic photosensitive members and functionally separated layered organic photosensitive members are the photosensitive members with potential decaying characteristics. All of the above are in this category.
However, in recent years, the necessity for improvements in high image quality, high additive values and networking have led to great steps in research. In digital mode electrophotographic copying apparatus, it is common that the area gradation system in which gradation is reproduced by dot area proportions is used. Accordingly, as shown in FIG. 2, there is another type of photosensitive member in which potential decay does not occur until the exposure amount has reached a predetermined level, i.e., a photosensitive member having the so-called S-type photo-induced potential decaying characteristics. When this fixed exposure amount is exceeded, sudden potential decay occurs (hereinafter referred to as an S-type photosensitive member). Use of the so-called S-type photopotential decaying photosensitive members results in increase in sharpness of the pixels and other welcome improvements.
The phenomenon of the S-type photo-induced potential decaying properties is well known in the single layered photosensitive members in which inorganic pigments such as ZnO or other pigments such as phthalocyanine are particle dispersed into a resin. See R. M. Schaffert: Electrophotography, Focal Press, pp 342 (1975), J. W. Weigl, J.Mammino, G. L.Whittaker, R. W. Radler, J. F. Byrne: Current problems in electrophotography, Walter de Gruyter, pp 287 (1972).
In particular, single layered photosensitive members for laser exposure which are sensitive to wavelengths of the near infra red region which corresponds to the oscillating wavelengths of semiconductors being widely used, have been suggested. See Gwen Chan Kay, Aizawa: Bulletin of Chemical Society of Japan, pp. 393 (1986), Japanese Patent Application Laid-Open(JP-A) Nos. 1-169454, 2-207258, 3-31847 and 5-313387.
However, in these single layered photosensitive members, it is necessary to have a single material which can function both as a charge generator and as a charge transporter. However, materials capable of performing both these functions are extremely rare, none of them is satisfactory in practice. In particular, the pigment particles generally possess many trap levels and this results in reduced charge transport ability. They also suffer from residual potential and are thus unsuitable charge transporters. The only exception is a single layered photosensitive member in which ZnO is dispersed in resin. In this instance, a printing plate is prepared by utilizing the hydrophilic properties of ZnO and the degree of adhesion of the hydrophobic toner to ZnO to obtain area gradation so that the sensitive member is used for a master plate for offset printing. See Kawamura: Electrophotographic techniques; rudiments and applications. Edited by Society of Electrophotography, Corona Publishing Co., pp. 424 (1988).
However, in this case, the sensitive member was used for a master plate which has low demands for high speed and high printing durability. In the field of use of the present invention, the photosensitive members used by copiers, printers, and other machines are not up to the required level of usability. From these points of view, in order to increase the freedom of choice of the materials for S-type photosensitive members and to increase the properties of the photosensitive members totally, the introduction of functionally separated layers is desired.
Regarding this problem, D. M. Pai et al. found that in a layered photosensitive member made up of the charge transport layer and the charge generation layer, the charge transport layer contains at least two charge transport regions and one electrically inactive region. The charge transport regions are in physical contact with each other so as to form a contorted charge transport path. Through the use of a heterogeneous charge transport layer comprising the contorted charge transport path and the combination of the charge generation layer with the transport layer, the S-type photo-induced potential decaying characteristics can be realized. See Japanese Patent Application Laid-Open (JP-A) No. 6-83077 and U. S. Pat. No. 5,306,586.
According to this report, the following methods of manufacturing heterogeneous charge transport layers formed by the contorted charge transport path in which charge transport regions are in contact with each other have been proposed: a manufacturing method of a heterogeneous charge transport layer in which organic or inorganic particles, or fine crystals capable of transporting charges are embedded in an insulating polymer; a manufacturing method in which a charge transport layer is made from a solid solution of charge transport molecules in a polymer binder, and, for example, one of the phases is crystallized; and a manufacturing method of a heterogeneous charge transport layer made by a block copolymer in which a charge transport block is encompassed by a charge non-transport block.
However, inorganic or organic particles, or crystallites capable of charge transporting normally possess many trap levels and thus when organic or inorganic particles or crystallites capable of charge transporting are embedded in an insulating polymer to make a heterogeneous charge transporting layer, the charge transporting ability is low or residual potential and other problems arise.
In addition, in a manufacturing method in which charge transport layers are made from solid solutions of charge transport molecules in polymer binders, and one of the phases is crystallized, compounds fulfilling the above conditions are rare. Accordingly, designing a photosensitive member capable of withstanding real usage is problematic.
Furthermore, a charge transport layer made from a block polymer enveloped in a charge non-transport block poses none of the above problems. However, it is difficult to maintain characteristics such as increased charge transport speed, reduction of residual potential and improved reproducing stability, and S-type properties at the same time.
By increasing the number of charge transport blocks to maintain the charge transport paths, the S-type properties deteriorate. By decreasing charge transport blocks to improve S-type properties, the charge transport paths get separated leading to reduced charge transport speed, increased residual potential, and less reproducing stability. Finding a photosensitive member which satisfies the above described characteristics at the same time was very difficult indeed.